<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/1998/REC-html40-19980424/loose.dtd">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Native C Calls</title>
<link rel="stylesheet" href="doc.css" type="text/css">
</head>
<body>
<a href="mailto:abu@software-lab.de">abu@software-lab.de</a>

<h1>Native C Calls</h1>

<p align=right>(c) Software Lab. Alexander Burger

<p>This document describes how to call C functions in shared object files
(libraries) from PicoLisp, using the built-in <code><a
href="refN.html#native">native</a></code> function - possibly with the help of
the <code><a href="refS.html#struct">struct</a></code> and <code><a
href="refL.html#lisp">lisp</a></code> functions. It applies only to the 64-bit
version of PicoLisp.

<p><ul>
<li><a href="#overview">Overview</a>
<li><a href="#syntax">Syntax</a>
   <ul>
   <li><a href="#libs">Libraries</a>
   <li><a href="#funs">Functions</a>
   <li><a href="#retval">Return Value</a>
      <ul>
      <li><a href="#primRet">Primitive Types</a>
      <li><a href="#structRet">Arrays and Structures</a>
      </ul>
   <li><a href="#args">Arguments</a>
      <ul>
      <li><a href="#primArg">Primitive Types</a>
      <li><a href="#structArg">Arrays and Structures</a>
      </ul>
   </ul>
<li><a href="#memory">Memory Management</a>
   <ul>
   <li><a href="#fftw">Fast Fourier Transform</a>
   <li><a href="#const">Constant Data</a>
   </ul>
<li><a href="#callbacks">Callbacks</a>
   <ul>
   <li><a href="#byName">Call by Name</a>
   <li><a href="#funptr">Function Pointer</a>
   </ul>
</ul>


<p><hr>
<h2><a name="overview">Overview</a></h2>

<p><code>native</code> calls a C function in a shared library. It tries to

<p><ol>
<li>find a library by name
<li>find a function by name in the library
<li>convert the function's argument(s) from Lisp to C structures
<li>call the function's C code
<li>convert the function's return value(s) from C to Lisp structures
</ol>

<p>The direct return value of <code>native</code> is the Lisp representation of
the C function's return value. Further values, returned by reference from the C
function, are available in Lisp variables (symbol values).

<p><code>struct</code> is a helper function, which can be used to manipulate C
data structures in memory. It may take a scalar (a numeric representation of a C
value) to convert it to a Lisp item, or (more typically) a pointer to a memory
area to build and extract data structures. <code>lisp</code> allows you to
install callback functions, callable from C code, written in Lisp.

<p>In combination, these three functions can interface PicoLisp to almost any C
function.

<p>The above steps are fully dynamic; <code>native</code> doesn't have (and
doesn't require) a priory knowledge about the library, the function or the
involved data. No need to write any glue code, interfaces or include files. All
functions can even be called interactively from the REPL.


<p><hr>
<h2><a name="syntax">Syntax</a></h2>

<p>The arguments to <code>native</code> are

<p><ol>
<li>a library
<li>a function
<li>a return value specification
<li>optional arguments
</ol>

<p>The simplest form is a call to a function without return value and without
arguments. If we assume a library "lib.so", containing a function with the
prototype

<pre><code>
void fun(void);
</code></pre>

<p>then we can call it as

<pre><code>
(native "lib.so" "fun")
</code></pre>


<p><hr>
<h3><a name="libs">Libraries</a></h3>

<p>The first argument to <code>native</code> specifies the library. It is either
the <i>name</i> of a library (a symbol), or the <i>handle</i> of a previously
found library (a number).

<p>As a special case, a transient symbol <code>"@"</code> can be passed for the
library name. It then refers to the current main program (instead of an external
library), and can be used for standard functions like <code>"malloc"</code> or
<code>"printf"</code>.

<p><code>native</code> uses <code>dlopen(3)</code> internally to find and open
the library, and to obtain the handle. If the name contains a slash ('/'), then
it is interpreted as a (relative or absolute) pathname. Otherwise, the dynamic
linker searches for the library according to the system's environment and
directories. See the man page of <code>dlopen(3)</code> for further details.

<p>If called with a symbolic argument, <code>native</code> automatically caches
the handle of the found library in the value of that symbol. The most natural
way is to pass the library name as a <a href="ref.html#transient">transient</a>
symbol (<code>"lib.so"</code> above): The initial value of a transient symbol is
that symbol itself, so that <code>native</code> receives the library name upon
the first call. After successfully finding and opening the library,
<code>native</code> stores the handle of that library in the value of the passed
symbol (<code>"lib.so"</code>). As <code>native</code> evaluates its arguments
in the normal way, subsequent calls within the same transient scope will receive
the numeric value (the handle), and don't need to open and search the library
again.


<p><hr>
<h3><a name="funs">Functions</a></h3>

<p>The same rules applies to the second argument, the function. When called with
a symbol, <code>native</code> stores the function pointer in its value, so that
subsequent calls evaluate to that pointer, and <code>native</code> can directly
jump to the function.

<p><code>native</code> uses <code>dlsym(3)</code> internally to obtain the
function pointer. See the man page of <code>dlsym(3)</code> for further details.

<p>In most cases a program will call more than one function from a given
library. If we keep the code within the same transient scope (i.e. in the same
source file, and not separated by the <code><a
href="ref_.html#====">====</a></code> function), each library will be opened -
and each function searched - only once.

<pre><code>
(native "lib.so" "fun1")
(native "lib.so" "fun2")
(native "lib.so" "fun3")
</code></pre>

<p>After <code>"fun1"</code> was called, <code>"lib.so"</code> will be open, and
won't be re-opened for <code>"fun2"</code> and <code>"fun3"</code>. Consider
the definition of helper functions:

<pre><code>
(de fun1 ()
   (native "lib.so" "fun1") )

(de fun2 ()
   (native "lib.so" "fun2") )

(de fun3 ()
   (native "lib.so" "fun3") )
</code></pre>

<p>After any one of <code>fun1</code>, <code>fun2</code> or <code>fun3</code>
was called, the symbol <code>"lib.so"</code> will hold the library handle. And
each function function <code>"fun1"</code>, <code>"fun2"</code> and
<code>"fun3"</code> will be searched only when called the first time.

<p>Warning: It should be avoided to put more than one library into a single
transient scope if there is a chance that two different functions with the same
name will be called in two different libraries. Because of the function pointer
caching, the second call would otherwise (wrongly) go to the first function.



<p><hr>
<h3><a name="retval">Return Value</a></h3>

<p>The (optional) third argument to <code>native</code> specifies the return
value. A C function can return many types of values, like integer or floating
point numbers, string pointers, or pointers to structures which in turn consist
of those types, and even other structures or pointers to structures.
<code>native</code> tries to cover most of them.

<p>As described in the <a href="refN.html#natResult">result specification</a>,
the third argument should consist of a pattern which tells <code>native</code>
how to extract the proper value.


<h4><a name="primRet">Primitive Types</a></h4>

<p>In the simplest case, the result specification is <code>NIL</code> like in
the examples so far. This means that either the C function returns
<code>void</code>, or that we are not interested in the value. The return value
of <code>native</code> will be <code>NIL</code> in that case.

<p>If the result specification is one of the symbols <code>B</code>,
<code>I</code> or <code>N</code>, an integer number is returned, by interpreting
the result as a <code>char</code> (8 bit unsigned byte), <code>int</code> (32
bit signed integer), or <code>long</code> number (64 bit signed integer),
respectively. Other (signed or unsigned numbers, and of different sizes) can be
produced from these types with logical and arithmetic operations if necessary.

<p>If the result specification is the symbol <code>C</code>, the result is
interpreted as a 16 bit number, and a single-char transient symbol (string) is
returned.

<p>A specification of <code>S</code> tells <code>native</code> to interpret the
result as a pointer to a C string (null terminated), and to return a transient
symbol (string).

<p>If the result specification is a number, it will be used as a scale to
convert a returned <code>double</code> (if the number is positive) or
<code>float</code> (if the number is negative) to a scaled fixpoint number.

<p>Examples for function calls, with their corresponding C prototypes:

<pre><code>
(native "lib.so" "fun" 'I)             # int fun(void);
(native "lib.so" "fun" 'N)             # long fun(void);
(native "lib.so" "fun" 'N)             # void *fun(void);
(native "lib.so" "fun" 'S)             # char *fun(void);
(native "lib.so" "fun" 1.0)            # double fun(void);
</code></pre>


<h4><a name="structRet">Arrays and Structures</a></h4>

<p>If the result specification is a list, it means that the C function returned
a pointer to an array, or an arbitrary memory structure. The specification list
should then consist of either the above primitive specifications (symbols or
numbers), or of cons pairs of a primitive specification and a repeat count, to
denote arrays of the given type.

<p>Examples for function calls, with their corresponding pseudo C prototypes:

<pre><code>
(native "lib.so" "fun" '(I . 8))       # int *fun(void);  // 8 integers
(native "lib.so" "fun" '(B . 16))      # unsigned char *fun(void);  // 16 bytes

(native "lib.so" "fun" '(I I))         # struct {int i; int j;} *fun(void);
(native "lib.so" "fun" '(I . 4))       # struct {int i[4];} *fun(void);

(native "lib.so" "fun" '(I (B . 4)))   # struct {
                                       #    int i;
                                       #    unsigned char c[4];
                                       # } *fun(void);

(native "lib.so" "fun"                 # struct {
   '(((B . 4) I) (S . 12) (N . 8)) )   #    struct {unsigned char c[4]; int i;}
                                       #    char *names[12];
                                       #    long num[8];
                                       # } *fun(void);
</code></pre>

<p>If a returned structure has an element which is a <i>pointer</i> to some
other structure (i.e. not an embedded structure like in the last example above),
this pointer must be first obtained with a <code>N</code> pattern, which can
then be passed to <code><a href="refS.html#struct">struct</a></code> for further
extraction.


<p><hr>
<h3><a name="args">Arguments</a></h3>

<p>The (optional) fourth and following arguments to <code>native</code> specify
the arguments to the C function.


<h4><a name="primArg">Primitive Types</a></h4>

<p>Integer arguments (up to 64 bits, signed or unsigned <code>char</code>,
<code>short</code>, <code>int</code> or <code>long</code>) can be passed as they
are: As numbers.

<pre><code>
(native "lib.so" "fun" NIL 123)        # void fun(int);
(native "lib.so" "fun" NIL 1 2 3)      # void fun(int, long, short);
</code></pre>

<p>String arguments can be specified as symbols. <code>native</code> allocates
memory for each string (with <code>strdup(3)</code>), passes the pointer to the
C function, and releases the memory (with <code>free(3)</code>) when done.

<pre><code>
(native "lib.so" "fun" NIL "abc")      # void fun(char*);
(native "lib.so" "fun" NIL 3 "def")    # void fun(int, char*);
</code></pre>

<p>Note that the allocated string memory is released <i>after</i> the return
value is extracted. This allows a C function to return the argument string
pointer, perhaps after modifying the data in-place, and receive the new string
as the return value (with the <code>S</code> specification).

<pre><code>
(native "lib.so" "fun" 'S "abc")       # char *fun(char*);
</code></pre>

<p>Also note that specifying <code>NIL</code> as an argument passes an empty
string ("", which also reads as <code>NIL</code> in PicoLisp) to the C function.
Physically, this is a pointer to a NULL-byte, and is <u>not</u> a NULL-pointer.
Be sure to pass <code>0</code> (the number zero) if a NULL-pointer is desired.

<p>Floating point arguments are specified as cons pairs, where the value is in
the CAR, and the CDR holds the fixpoint scale. If the scale is positive, the
number is passed as a <code>double</code>, otherwise as a <code>float</code>.

<pre><code>
(native "lib.so" "fun" NIL             # void fun(double, float);
   (12.3 . 1.0) (4.56 . -1.0) )
</code></pre>


<h4><a name="structArg">Arrays and Structures</a></h4>

<p>Composite arguments are specified as nested list structures.
<code>native</code> allocates memory for each array or structure (with
<code>malloc(3)</code>), passes the pointer to the C function, and releases the
memory (with <code>free(3)</code>) when done.

<p>This implies that such an argument can be both an input and an output value
to a C function (pass by reference).

<p>The CAR of the argument specification can be <code>NIL</code> (then it is an
input-only argument). Otherwise, it should be a variable which receives the
returned structure data.

<p>The CADR of the argument specification must be a cons pair with the total
size of the structure in its CAR. The CDR is ignored for input-only arguments,
and should contain a <a href="refN.html#natResult">result specification</a> for
the output value to be stored in the variable.

<p>For example, a minimal case is a function that takes an integer reference,
and stores the number '123' in that location:

<pre><code>
void fun(int *i) {
   *i = 123;
}
</code></pre>

<p>We call <code>native</code> with a variable <code>X</code> in the CAR of the
argument specification, a size of 4 (i.e. <code>sizeof(int)</code>), and
<code>I</code> for the result specification. The stored value is then available
in the variable <code>X</code>:

<pre><code>
: (native "lib.so" "fun" NIL '(X (4 . I)))
-> NIL
: X
-> 123
</code></pre>

<p>The rest (CDDR) of the argument specification may contain initialization
data, if the C function expects input values in the structure. It should be a
list of <a href="refN.html#natItem">initialization items</a>, optionally with a
fill-byte value in the CDR of the last cell.

<p>If there are <i>no</i> initialization items and just the final fill-bye, then
the whole buffer is filled with that byte. For example, to pass a buffer of 20
bytes, initialized to zero:

<pre><code>
: (native "lib.so" "fun" NIL '(NIL (20) . 0))
</code></pre>

<p>A buffer of 20 bytes, with the first 4 bytes initialized to 1, 2, 3, and 4,
and the rest filled with zero:

<pre><code>
: (native "lib.so" "fun" NIL '(NIL (20) 1 2 3 4 . 0))
</code></pre>

<p>and the same, where the buffer contents are returned as a list of bytes in
the variable <code>X</code>:

<pre><code>
: (native "lib.so" "fun" NIL '(X (20 B . 20) 1 2 3 4 . 0))
</code></pre>

<p>For a more extensive example, let's use the following definitions:

<pre><code>
typedef struct value {
   int x, y;
   double a, b, c;
   int z;
   char nm[4];
} value;

void fun(value *val) {
   printf("%d %d\n", val->x, val->y);
   val->x = 3;
   val->y = 4;
   strcpy(val->nm, "OK");
}
</code></pre>

<p>We call this function with a structure of 40 bytes, requesting the returned
data in <code>V</code>, with two integers <code>(I . 2)</code>, three doubles
<code>(100 . 3)</code> with a scale of 2 (1.0 = 100), another integer
<code>I</code> and four characters <code>(C . 2)</code>. If the structure gets
initialized with two integers 7 and 6, three doubles 0.11, 0.22 and 0.33, and
another integer 5 while the rest of the 40 bytes is cleared to zero

<pre><code>
: (native "lib.so" "fun" NIL
   '(V (40 (I . 2) (100 . 3) I (C . 4)) -7 -6 (100 11 22 33) -5 . 0) )
</code></pre>

<p>then it will print the integers 7 and 6, and <code>V</code> will contain the
returned list

<pre><code>
((3 4) (11 22 33) 5 ("O" "K" NIL NIL))
</code></pre>

<p>i.e. the original integer values 7 and 6 replaced with 3 and 4.

<p>Note that the allocated structure memory is released <i>after</i> the return
value is extracted. This allows a C function to return the argument structure
pointer, perhaps after modifying the data in-place, and receive the new
structure as the return value - instead of (or even in addition to) to the
direct return via the argument reference.


<p><hr>
<h2><a name="memory">Memory Management</a></h2>

<p>The preceding <a href="#args">Arguments</a> section mentions that
<code>native</code> implicitly allocates and releases memory for strings, arrays
and structures.

<p>Technically, this mimics <i>automatic variables</i> in C.

<p>For a simple example, let's assume that we want to call <code>read(2)</code>
directly, to fetch a 4-byte integer from a given file descriptor. This could be
done with the following C function:

<pre><code>
int read4bytes(int fd) {
   char buf[4];

   read(fd, buf, 4);
   return *(int*)buf;
}
</code></pre>

<p><code>buf</code> is an automatic variable, allocated on the stack, which
disappears when the function returns. A corresponding <code>native</code> call
would be:

<pre><code>
(native "@" "read" 'N Fd '(Buf (4 . I)) 4)
</code></pre>

<p>The structure argument <code>(Buf (4 . I))</code> says that a space of 4
bytes should be allocated and passed to <code>read</code>, then an integer
<code>I</code> returned in the variable <code>Buf</code> (the return value of
<code>native</code> itself is the number returned by <code>read</code>). The
memory space is released after that.

<p>(Note that we use <code>"@"</code> for the library here, as <code>read</code>
resides in the main program.)

<p>Instead of a single integer, we might want a list of four bytes to be
returned from <code>native</code>:

<pre><code>
(native "@" "read" 'N Fd '(Buf (4 B . 4)) 4)
</code></pre>

<p>The difference is that we wrote <code>(B . 4)</code> (a list of 4 bytes)
instead of <code>I</code> (a single integer) for the <a
href="refN.html#natResult">result specification</a> (see the <a
href="#structArg">Arrays and Structures</a> section).

<p>Let's see what happens if we extend this example. We'll write the four bytes
to another file descriptor, after reading them from the first one:

<pre><code>
void copy4bytes(int fd1, int fd2) {
   char buf[4];

   read(fd1, buf, 4);
   write(fd2, buf, 4);
}
</code></pre>

<p>Again, <code>buf</code> is an automatic variable. It is passed to both
<code>read</code> and <code>write</code>. A direct translation would be:

<pre><code>
(native "@" "read" 'N Fd '(Buf (4 B . 4)) 4)
(native "@" "write" 'N Fd (cons NIL (4) Buf) 4)
</code></pre>

<p>This work as expected. <code>read</code> returns a list of four bytes in
<code>Buf</code>. The call to <code>cons</code> builds the structure

<pre><code>
(NIL (4) 1 2 3 4)
</code></pre>

<p>i.e. no return variable, a four-byte memory area, filled with the four bytes
(assuming that <code>read</code> returned 1, 2, 3 and 4). Then this structure is
passed to <code>write</code>.

<p>But: This solution induces quite some overhead. The four-byte buffer is
allocated before the call to <code>read</code> and released after that, then
allocated and released again for <code>write</code>. Also, the bytes are
converted to a list to be stored in <code>Buf</code>, then that list is extended
for the structure argument to <code>write</code>, and converted again back to
the raw byte array. The data in the list itself are never used.

<p>If the above operation is to be used more than once, it is better to allocate
the buffer manually, use it for both reading and writing, and then release it.
This also avoids all intermediate list conversions.

<pre><code>
(let Buf (native "@" "malloc" 'N 4) # Allocate memory
   (native "@" "read" 'N Fd Buf 4)  # (Possibly repeat this several times)
   (native "@" "write" 'N Fd Buf 4)
   (native "@" "free" NIL Buf) )    # Release memory
</code></pre>


<h4><a name="fftw">Fast Fourier Transform</a></h4>

<p>For a more typical example, we might call the Fast Fourier Transform using
the library from the <a href="http://fftw.org">FFTW</a> package. With the
example code for calculating Complex One-Dimensional DFTs:

<pre><code>
#include &lt;fftw3.h&gt;
...
{
   fftw_complex *in, *out;
   fftw_plan p;
   ...
   in = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);
   out = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * N);
   p = fftw_plan_dft_1d(N, in, out, FFTW_FORWARD, FFTW_ESTIMATE);
   ...
   fftw_execute(p); /* repeat as needed */
   ...
   fftw_destroy_plan(p);
   fftw_free(in); fftw_free(out);
}
</code></pre>

<p>we can build the following equivalent:

<pre><code>
(load "@lib/math.l")

(de FFTW_FORWARD . -1)
(de FFTW_ESTIMATE . 64)

(de fft (Lst)
   (let
      (Len (length Lst)
         In (native "libfftw3.so" "fftw_malloc" 'N (* Len 16))
         Out (native "libfftw3.so" "fftw_malloc" 'N (* Len 16))
         P (native "libfftw3.so" "fftw_plan_dft_1d" 'N
            Len In Out FFTW_FORWARD FFTW_ESTIMATE ) )
      (struct In NIL (cons 1.0 (apply append Lst)))
      (native "libfftw3.so" "fftw_execute" NIL P)
      (prog1
         (struct Out (make (do Len (link (1.0 . 2)))))
         (native "libfftw3.so" "fftw_destroy_plan" NIL P)
         (native "libfftw3.so" "fftw_free" NIL Out)
         (native "libfftw3.so" "fftw_free" NIL In) ) ) )
</code></pre>

<p>This assumes that the argument list <code>Lst</code> is passed as a list
of complex numbers, each as a list of two numbers for the real and imaginary
part, like

<pre><code>
(fft '((1.0 0) (1.0 0) (1.0 0) (1.0 0) (0 0) (0 0) (0 0) (0 0)))
</code></pre>

<p>The above translation to Lisp is quite straightforward. After the two buffers
are allocated, and a plan is created, <code><a
href="refS.html#struct">struct</a></code> is called to store the argument list
in the <code>In</code> structure as a list of double numbers (according to the
<code>1.0</code> <a href="refN.html#natItem">initialization item</a>). Then
<code>fftw_execute</code> is called, and <code>struct</code> is called again to
retrieve the result from <code>Out</code> and return it from <code>fft</code>
via the <code><a href="refP.html#prog1">prog1</a></code>. Finally, all memory is
released.


<h4><a name="const">Constant Data</a></h4>

<p>If such allocated data (strings, arrays or structures passed to
<code>native</code>) are constant during the lifetime of a program, it makes
sense to allocate them only once, before their first use. A typical candidate is
the format string of a <code>printf</code> call. Consider a function which
prints a floating point number in scientific notation:

<pre><code>
(load "@lib/math.l")

: (de prf (Flt)
   (native "@" "printf" NIL "%e^J" (cons Flt 1.0)) )
-> prf

: (prf (exp 12.3))
2.196960e+05
</code></pre>

<p>As we know that the format string <code>"%e^J"</code> will be converted from
a Lisp symbol to a C string with <code>strdup</code> - and then thrown away - on
each call to <code>prf</code>, we might as well perform a little optimization
and delegate this conversion to the program load time:

<pre><code>
: (de prf (Flt)
   (native "@" "printf" NIL `(native "@" "strdup" 'N "%e^J") (cons Flt 1.0)) )
-> prf

: (prf (exp 12.3))
2.196960e+05
</code></pre>

<p>If we look at the <code>prf</code> function, we see that it now contains the
pointer to the allocated string memory:

<pre><code>
: (pp 'prf)
(de prf (Flt)
   (native "@" "printf" NIL 24662032 (cons Flt 1000000)) )
-> prf
</code></pre>

<p>This pointer will be used by <code>printf</code> directly, without any
further conversion or memory management.


<p><hr>
<h2><a name="callbacks">Callbacks</a></h2>

<p>Sometimes it is necessary to do the reverse: Call Lisp code from C code. This
can be done in two ways - with certain limitations.


<h4><a name="byName">Call by Name</a></h4>

<p>The first way is actually not a <a
href="http://en.wikipedia.org/wiki/Callback_(computer_programming)">callback</a>
in the strict sense. It just allows to call a Lisp function with a given name.

<p>The limitation is that this function can accept only maximally five numeric
arguments, and returns a number.

<p>The prerequisite is, of course, that you have access to the C source code. To
use it from C, insert the following prototype somewhere before the first call:

<pre><code>
long lisp(char*,long,long,long,long,long);
</code></pre>

<p>Then you can call <code>lisp</code> from C:

<pre><code>
long n = lisp("myLispFun", a, b, 0, 0, 0);
</code></pre>

<p>The first argument should be the name of a Lisp function (built-in, or
defined in Lisp). It is searched for at runtime, so it doesn't need to exist at
the time the C library is compiled or loaded.

<p>Be sure to pass dummy arguments (e.g. zero) if your function expects less
than five arguments, to keep the C compiler happy.

<p>This mechanism can generally be used for any type of argument and return
value (not only <code>long</code>). On the C side, appropriate casts or a
adapted prototype should be used. It is then up to the called Lisp function to
prepare and/or extract the proper data with <code><a
href="refS.html#struct">struct</a></code> and memory management operations.


<h4><a name="funptr">Function Pointer</a></h4>

<p>This is a true <a
href="http://en.wikipedia.org/wiki/Callback_(computer_programming)">callback</a>
mechanism. It uses the Lisp-level function <code><a
href="refL.html#lisp">lisp</a></code> (not to confuse with the C-level function
with the same name in the previous section). No C source code access is
required.

<p><code>lisp</code> returns a function pointer, which can be passed to C
functions via <code>native</code>. When this function pointer is dereferenced
and called from the C code, the corresponding Lisp function is invoked. Here,
too, only five numeric arguments and a numeric return value can be used, and
other data types must be handled by the Lisp function with <code><a
href="refS.html#struct">struct</a></code> and memory management operations.

<p>Callbacks are often used in user interface libraries, to handle key-, mouse-
and other events. Examples can be found in <code>"@lib/openGl.l"</code>. The
following function <code>mouseFunc</code> takes a Lisp function, installs it
under the tag <code>mouseFunc</code> (any other tag would be all right too) as a
callback, and passes the resulting function pointer to the OpenGL
<code>glutMouseFunc()</code> function, to set it as a callback for the current
window:

<pre><code>
(de mouseFunc (Fun)
   (native `*GlutLib "glutMouseFunc" NIL (lisp 'mouseFunc Fun)) )
</code></pre>

<p>(The global <code>*GlutLib</code> holds the library
<code>"/usr/lib/libglut.so"</code>. The backquote (<code>`</code>) is important
here, so that the transient symbol with the library name (and not the global
<code>*GlutLib</code>) is evaluated by <code>native</code>, resulting in the
proper library handle at runtime).

<p>A program using OpenGL may then use <code>mouseFunc</code> to install a
function

<pre><code>
(mouseFunc
   '((Btn State X Y)
      (do-something-with Btn State X Y) ) )
</code></pre>

<p>so that future clicks into the window will pass the button, state and
coordinates to that function.

</body>
</html>
